U.S. Pat. No. 4,760,420, which issued on Jul. 26, 1988 in the name of Lam, discloses a shutter mechanism for a camera having overlapping plates which are pivotally mounted and connected by a pin sliding in slots in the plates so that the plates move in unison. Springs bias the plates to an overlapping position, covering a hole in the camera body which leads to a lens. One of the plates is struck by a lever which urges the plates against springs causing them to pivot away, until the lever strikes a stop at which point the plates still overlap sufficiently to cover the hole. After the lever strikes the stop the plates continue to move under their own momentum to open up an aperture bound by the edges.
A magnetic control plate with pawls is attracted towards the overlapping plates by an electromagnet so that the pawls engage racks on the plates to arrest the plates at a particular aperture setting. A light metering device or optical sensor is used to determine the position of the plates and thereby to control energization of the electromagnet. The inertia of the magnetic control plate requires that the electromagnet must be energized slightly before the desired aperture size is reached. The electromagnet is switched off, releasing the plates which close the aperture under the action of the springs.